A Novel Dual-Loop Control Strategy to Improve the Power Quality of Microgrids

Transcription

1 A Novel Dual-Loop Control Strategy to Improve the Power Qualty of Mcrogrs Lan ZHOU College of Electrcal & Informaton Engneerng, Hunan Insttute of Engneerng, Xangtan, Hunan, 44, P.R. Chna Abstract Ths paper presents a new power qualty ual-loop control strategy whch uses a fee-forwar f-p/v-q roop controller an a voltage-current controller for the slane operaton. The propose roop controller calculates the requre actve an reactve power of mcrosources usng errors of the frequency an the voltage ampltue, whch s useful for mprovng the response of the output power regulaton. The fee-forwar f-p/v-q roop controller forms the voltage control sgnal for the voltage-current controller whch employs approxmate ecouplng control to lghten the actve an reactve power ecouplng for the mcrosources an enhance the power control precson. Smulaton results emonstrate that ths propose control strategy has strong oscllaton suppress ablty an ynamc performance. The controller can also enhance the operaton capacty of the mcrogr. Keywors - Feeforwar systems; frequency control; nverters; mcrogr; power qualty; voltage control I. INTRODUCTION Wth the global warmng, many countres are focusng on cuttng ther epenence on fossl fuels an evelopng renewable energy [] [2]. Uner ths stuaton, the concept of "mcrogr" has been propose to mprove the ablty of power gr to accept volatlty strbute source [3] [4]. Mcrogrs can be operate parallel-connecte by connectng strbuton network to large power gr, whch s capable of meetng local loa eman nepenently. Mcrogrs can not only reuce the nfluence of tratonal gr cause by a large number of small strbute sources, enhance the effcency of energy utlzaton, but also mprove the securty an relablty of power system. In recent years, the herarchcal control metho for the mcrogrs has been stue [5-7]; two typcal roopng control methos use for mcrosources have been stue [9-]; a kn of vrtual mpeance metho whch uses Q-L characterstcs nstea of Q-V characterstcs to control the output voltage of nverter has been propose [2][3]; conserng the fact that the resstance of low voltage strbuton power lnes s mpeance, a P-V/Q-f control metho has been propose [4], whch uses the actve power to control voltage ampltue an uses the reactve power to control voltage frequency; a strategy for controllng a mcrogr uner three-phase unbalance stuaton has been propose [5]. A fee forwar control for strbute generatons has been ncate [6], whch shows that the outer loop fee forwar mproves system stablty. Many roopng control strategys focus on sharng the loa power for mcrosources an work very well, but most of them pay lttle attenton to the ynamc voltage an frequency sturbance of mcrogrs. Wth the use of a large number of precson equpments, as well as the evelopment of smart gr, the users are n great nee of hgher power qualty. Though, a mcrogr can meet the unnterrupte power supply nees of users, t shoul prove users wth hgh-qualty power supply to cater ther nees n the smart gr. Furthermore, for the Consortum for Electrc Relablty Technology Solutons (CERTS) mcrogr, a crtcal component whch name statc swtch s neee. The gr requres a lot of preparaton tme when t reconnect to the utlty gr because the approprate frequency, voltage ampltue an phase angle requrements must be met an the typcal roopng control metho oes not follow the frequency an voltage ampltue [7]. If the mcrogr can follow the utlty gr voltage change at any tme, then the preparaton tme an the nvestment costs of the harware wll be reuce. Base on the analyss above, a power qualty ual-loop control strategy for mcrogrs has been propose n ths paper. It can restran oscllatons n the mcrogr when the mcrosources an loas change suenly, an s also useful to ecrease the mpact current when the mcrogr s connecte to a strbuton network. II. THE OVERALL THOUGHT OF MICROGRID S POWER QUALITY To solve problems of the voltage fluctuatons an frequency evaton exstng n mcrogrs, ths paper carres out a strategy about power qualty ual-loop control of mcrogrs, as Fgure shows. Frstly, t can calculates reactve an actve power that the gr an compoun voltage control sgnal nees through the fee-forwar f-p/v-q roopng controller by makng the use of the fferent value between the frequency an voltage ampltue. The calculaton helps mprovng the precson an response of the output power of mcrosources. Seconly, t aopts an approxmate ecouplng control strategy to ecrease the couplng between output actve an reactve power of the nverter through a capactor voltage controller (outer loop) an an nuctor current controller (nner loop). Ltter output of the actve an reactve power s requre. So the shock of the mcrogr can be prevente effectvely when the mcrosources or loas change ramatcally. It s more secure an stable to operate the gr than other control strategys. Ths strategy can quckly follow the real-tme voltage sgnal DOI.53/IJSSST.a ISSN: x onlne, prnt

2 of the strbute network an avo mpulse current when the gr connecte suenly.. Fgure. The schematc agram of the fee-forwar f-p/ V-Q roopng control metho.. III. THE FEED-FORWARD F-P/V-Q DROOPING CONTROLLER Fgure 2. Fgure 3. P-ω Droopng curve Q-V Droopng curve. Fgure 2 an Fgure 3 show the characterstc curve of the mcrosource. ω s the set-pont of frequency, ω s the rate frequency an p s the rate output power of the mcrosource, p s the actual loa power, m s the roopng curve. It can be euce as follows: m( P P) () If there s an frequency error between the rate frequency of the mcrogr ω ref (ω ref = ω ) an the real-tme frequency of the mcrogr, then: ref (2) The actve output power of the mcrosource s change by equaton: P kp (3) t From (-3) the frequency ynamc error s: ( ) P m mkp r( ) (4) t t kmp rt C () t e (5) t () C t Pl e (6) n m j j C (7) km P r DOI.53/IJSSST.a ISSN: x onlne, prnt

3 It can be seen from (5) an (6) that the mcrogr frequency error wll fnally sappear, the frequency of the mcrogr wll be steay n the mcrogr settng frequency. The change rate s ece by the constant C, an the constant C s ece by varable k. It can quckly elmnate the mcrogr frequency error an balance the system actve power by settng reasonable parameters. The connecton transent between the mcrogr an utlty gr s more senstve to phase msmatch an frequency msmatch than magntue msmatch. The roop characterstc of the mcrosource s shftte vertcally by a small amount when changng the phase of the mcrogr voltage slghtly, Assumng that there s a vrtual roopng controlle mcrosource at pont A n Fgure. The roops are now specfe as follows: B mb( PB PB) (8) The roops of the vrtual roopng controlle mcrosource s: A ma( PA PA) (9) At the start of the synchronzaton process, f ωref = ω s the rate frequency of the utlty gr, then: ref B mb ( P B PB ) () t 2 PB E ( B A) cost () t L δt s the angle between eabc an Vabc, assumng that δt approxmately equal to zero, then equaton () can be further smplfe for δt, then: t m ( C A e ) (2) t mb ma L C (3) E 2 ( m A m B) Equaton (2) shows that the msmatch angle of phase ecreases wth tme. The same ervaton of the roop V-Q curve can be obtane: V V m ( Q Q) (4) V Vref V (5) Q kq V (6) t From (4) an (5) the voltage ampltue ynamc error s: ( V) Q m mkq ( V) (7) t t kmq t C V () t e (8) t V() C t Ql e n (9) m j j Fgure 4. C (2) km Q Block agram of fee-forwar f-p/v-q controller. The controller structure s shown n Fgure 4. ω ref s the settng frequency of the mcrogr, ω s the real-tme bus voltage frequency, P s the actve output power of the nverter; V ref s the settng voltage ampltue of the mcrogr, V s the real-tme bus voltage ampltue; Q s the reactve output power of the nverter. k p an k q are feeback gans of the f-p/v-q controller. The parameters K wp an K w are scale coeffcent an Integral gan coeffcent of the f-p controller respectvely, K Vp an K V are scale coeffcent an Integral gan coeffcent of the V-Q controller respectvely. Compare wth prevous roopng control methos [6], the avance f-p/v-q roopng control metho as frequency an ampltue recovery segments to mprove the ynamc response by fee-forwar controller. Then the voltage reference sgnal of the nverter s calculate by a power rng. However, the ecouplng control s neee n the en of the controller to mprove control accuracy because the output power couplng together. IV. THE CAPACITOR VOLTAGE-INDUCTIVE CURRENT CONTROLLER DESIGN The outer voltage an nner current control loops have been scusse [6]. Ths paper gves a etale theoretcal ervaton about t n next secton. A sngle-phase parallel-connecte equvalent crcut of the nverter s shown n Fgure, where e abc s the nverter output voltage. In other wors, t s the voltage sgnal of the avance f-p/v-q roopng controller. V abc s the bus voltage, C s the capactor of the flter, L s the nuctance of the flter, R s the equvalent resstance of the flter, abc s the nverter output current, L s the nput loa current of the bus, C s the capactor current. So t s nferre by Krchhoff s law as follows: abc eabc L abcrvabc t L C (2) abc V abc C C t It can be nferre from: DOI.53/IJSSST.a ISSN: x onlne, prnt

4 e R L t t V L e q L R V q t t q t q V t V t q L C C q t V q t V t Lq (22) (23) Obvously, e an e q are transforme to C, Cq an, q by (23) n q coornate, an q are transforme to V an V q by (22) n q coornate. Equatons are obtane as follows: K I ( KP )( Vref V) CVq L S (24) K I q ( KP )( Vqref Vq) CV Lq S KI e ( K )( ) L V S KI e ( K )( ) L V S P ref q q P qref q q (25) aoptng maxmum power pont track control metho; Mcrosource 3 s compose of a photovoltac evce an an energy storage evce usng PQ control metho. The mcrogr system aopts 25kW as the power unt value, 38V as the bus voltage unt value. Actve an reactve power settng of mcrosource are both. unt value. The elvere power of mcrosource 2 s nfluence by the weather contons. The parameter of Temperature s set to 25, an lght ntensty s W/m 2. The actve power settng of mcrosource 3 s set to.4 pu an the reactve power settng of mcrosource 3 s set to. p.u. Loa s a 25kW, 25kVar constant power loa. Loa2 s a 5kW, 25kVar constant power loa. Loa3 s a 25kW constant power loa. Loa4 s a 5kW, 25kVar constant power loa. Loa5 s a 5kW, 25kVar constant power loa. The parameters of the power qualty ual-loop controller are gven n Table. The parameters of the f-p/v-q controller are accorng to the typcal I system of Semens best tunng. L ref V e V ref V C L C 23 e abc V q C q L V qref qref e q Lq Fgure 5. The schematc agram of capactor voltage-nuctor current controller Corresponngly, the schematc agram of capactor voltage (outer loop) an nuctor current (nner loop) controllers are shown n Fgure 5. K CP an K C are scale coeffcent an ntegral gan coeffcent of the current controller respectvely. K LP an K L are scale coeffcent an Integral gan coeffcent of the voltage controller respectvely. Both of the external voltage loop an nternal current loop aopt an approxmate ecouplng control strategy to ecrease the sturbance between output actve power an reactve power of nverter sgnfcantly. So the control accuracy s mprove. V. SIMULATION ANALYSIS To verfy the ablty of power qualty management technology for mcrogr-control, a mcrogr smulaton moel s establshe base on MATLAB shown n Fgure 6. Mcrosource s compose of a fuel cell an an energy storage evce, usng the fee-forwar f-p/v-q roopng control metho; Mcrosource 2 s a photovoltac evce V q Fgure 6. Typcal structure of mcrogr. TABLE. THE SIMULATION PARAMETERS ABOUT THE POWER QUALITY DUAL-LOOP CONTROL STRATEGY OF MICROGRID Controlle Parameter Controller Parameter r kp kq.5 Kwp KVp Kw KV mp -. mq -. KCP KLP KC. KL 2 L.5 C.5 Case : Mcrogr Startng an Response to Loa Swtchng At s, statc swtch s close. B2, B3, B4, K3, K4, K5, K6 an K8 are close. Lne2, lne 3 an lne 4 s connecte to the low voltage strbuton network. Meanwhle, mcrosource 2 an mcrosource 3 begn to work. B s sconnecte. K an K2 are close an mcrosource DOI.53/IJSSST.a ISSN: x onlne, prnt

5 operatons wth loa nepenently. At.2s, B s close. A mcrogr system s bult an connecte to the low voltage strbuton network as well as operate wth t. At.4s, K7 s close. Loa4 s connecte to the gr. The voltage of the mcrogr bus s llustrate n Fgure 7 an Fgure 8. The output power of mcrosources s shown n Fgure 4, Fgure 5 an Fgure the output of the mcrosource s cut own an the bus voltage rses slghtly an then keeps steay after one an a half power cycle, as Fgure shows Fgure 9. The voltage at the mcrogr bus n case 2 at.6s Fgure 7. The voltage at the mcrogr bus n case at.2s Fgure. The voltage at the mcrogr bus n case 2 at.8s Fgure 8. The voltage at the mcrogr bus n case at.4s As Fgure 7 shows, when the mcrogr system s bult an connecte to strbuton n.2s, the power of mcrosource slghtly ncreases an there s no voltage fluctuaton. When loa4 s gr connecte n.4s, as Fgure 8 shows, the output power of mcrosource rses raply an there s a slght voltage fluctuaton. However, uner the propose control, the mcrosources reman stable. Case 2: Transton to the Islane Moe an Response to Power Fluctuatng At.6s, statc swtch s opene. The mcrogr s sconnecte from the low voltage strbuton network an operate slane. At.8s, lght ntensty ncreases sgnfcantly to 3 W/m 2, whch ncreases the elvere power of the mcrosource 3. At s, K7 s opene an loa4 s sconnecte from the gr. As Fgure 9 ncates, when the mcrogr s operate slane n.6s, the power of mcrosource ncreases slghtly. There s no mpact on current, no voltage fluctuaton, an no mpulse current. Fgure shows that when the power of mcrosource 2 ncreases n.8s, mcrosource respons raply an cuts own the output. The power of mcrogr s balance quckly an there s no voltage fluctuaton. When loa4 s sconnecte from the gr n s, Fgure. The voltage at the mcrogr bus n case 2 at.s Case 3: Transton to the Gr-connecte Moe At.2s, the statc swtch s close. The mcrogr s connecte to the strbuton network agan Fgure 2. The voltage at the mcrogr bus n case 3 Fgure 2 shows that the mcrogr system s bult an connecte to the strbuton network when the statc swtch s close n.2s. There s no voltage fluctuaton. The current n statc swtch ncreases graually an reaches to ts maxmum after two power cycle. So t can avo proucng mpulse current, as shown n Fgure 3. DOI.53/IJSSST.a ISSN: x onlne, prnt

6 I/pu Fgure 3. The current of statc swtch. Mcrosource aopts fee-forwar f-p/v-q roopng control metho to balance the system power of the mcrogr. Because frequency assocates wth ampltue recovery, the mcrogr voltage frequency s lmte to a small range of fluctuatons as shown n Fgure 7. Beses, the mcrogr output power n the whole process s stable, voltage an frequency fluctuatons are small n strbuton network, as shown n Fgure 8. In the gr-connecte moe, the mcrogr termnal voltage s ctate by the rest of the strbuton network, an represents the power system frequency. The mcrogr connecte wth other unts contrbutes to the regulaton of the network voltage an frequency. Durng the whole process, the evaton of the mcrogr frequency s less than ±.2Hz an the maxmum fluctuaton of the gr bus voltage s less than ±.3%. Both of them meet the requrements of the mcrogr. Output Power/pu P Q Frequency/Hz Frequency/Hz Output Power/pu Fgure 6. The output power of mcrosource Fgure 7. Mcrogr voltage frequency Fgure 8. The low voltage strbuton network voltage frequency. P Q Fgure 4. The output power of mcrosource VI. CONCLUSION Output Power/pu Fgure 5. The output power of mcrosource 2 P Q A power qualty ual-loop control strategy for mcrogrs has been propose n ths paper, t s useful to solve the problems of voltage fluctuatons an frequency evaton cause by ramatc change of mcrosources or loas. Frst, a fee-forwar f-p/v-q roopng controller s esgne to mprove the control precson an response spee of the output power of mcrosources. Secon, a control strategy wth an external voltage loop an an nternal current loop s aopte to solve the couplng between the output actve power an reactve power of the nverter. The propose control strategy can effectvely prevent the shock of the mcrogr when mcrosources or loas change ramatcally. The securty an stablty of the mcrogr have been mprove. The control strategy can also quckly follow the changes of strbute network voltage an avoe the mpact current when the mcrogr s connecte to the strbuton network. DOI.53/IJSSST.a ISSN: x onlne, prnt

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